This invention relates to belt furnaces; and more particularly, it relates to methods and apparatus for establishing certain target temperature profiles in a workpiece as it passes through a belt furnace.
Belt furnaces are widely used by manufacturers of integrated circuits. Each furnace includes an elongated passageway along which a plurality of spaced apart heaters are disposed; and a belt moves from one end of the passageway to the other to thereby carry a workpiece sequentially past the heaters. Typically, the workpiece is an integrated circuit package in which a silicon die, or a lid, or a heatsink is being attached with an adhesive. Each of the heaters has its own individual thermostat; and so the workpiece can be subjected to a temperature profile which varies in a certain desired fashion along the passageway. This temperature profile is chosen such that the adhesive cures properly, and at the same time, thermally induced stresses in the package are maintained at a tolerable level.
However, the thermostat temperature settings for the heaters do not have a one-to-one correspondence with the temperature of the workpiece along the passageway; and, there are many technical reasons why this is so. For example, (1) the workpiece is moving, and it takes time for its temperature to change as it moves from one heater to another; (2) the amount of heat transferred to the workpiece depends on the size of the passageway and the velocity of air in the passageway; (3) a workpiece having a large mass will heat slower than a workpiece having a small mass; (4) a workpiece with a shiny surface will absorb less radiated heat than a workpiece with a dull surface; (5) a workpiece having a large contact area with the belt 13 will lose/gain more heat via conduction to/from the belt itself than a workpiece with a small contact area; and, (6) air temperature in the passageway varies in a nonlinear fashion midway between two consecutive heaters.
This then presents the problem of how to set the belt furnace thermostats in order to achieve the target temperature profile in the workpiece itself. In the prior art, this problem was addressed by a trial-and-error or guessing-game method. Using this method, an operator would make an initial guess, based on his training and experience, as to what the thermostat setting should be; and he would then set the thermostats accordingly. After the furnace temperatures at those settings had stabilized, the operator would send the workpiece through the furnace with a thermocouple attached to it to obtain an actual temperature profile of the workpiece. This actual temperature profile was then compared by the operator to the desired target temperature profile; and if their differences were too large, the operator would make another guess on how the thermostats should be set. These steps were repeated over and over until the actual temperature profile matched the target temperature profile within a certain allowable tolerance.
A major problem, however, with the above prior art method is that it takes too long. Each time the thermostat settings are changed, several hours (typically 6-7 hours) must pass before an actual temperature profile can be made. This time is needed to ensure that the furnace temperatures have stabilized. Thus, a single incorrect guess on what the thermostat setting should be adds 6-7 hours to the overall process. Some guesses will be too high while other guesses will be too low; and to arrive at the target profile typically takes more than a dozen guesses. Also, a single integrated circuit package usually requires several temperature profiles (i.e., one to attach the die, one to attach the lid, one to attach the heat sink); and the thermostat setting to achieve these profiles will be different for each different type of integrated circuit package.
Accordingly, a primary object of the invention is to provide a method and apparatus for achieving a target temperature profile in a workpiece as it passes through a belt furnace, which is substantially faster than the prior art.